The present invention relates to a method for controlling an electric machine by means of an inverter, in particular for use in a motor vehicle, wherein the inverter has a plurality of controllable power switches which are designed to convert a direct voltage of a voltage source, coupled to the power switches, into an alternating voltage for supplying the electric machine with electrical energy.
In addition, the present invention relates to an apparatus for actuating an electric machine by means of an inverter, in particular for use in a motor vehicle, wherein the inverter has a plurality of controllable power switches which are designed to convert a direct voltage of a voltage source, coupled to the power switches, into an alternating voltage for supplying the electric machine with electrical energy, having a control unit for controlling the inverter.
In addition, the present invention relates to a motor vehicle drive train having an electric machine for making available drive power and having an apparatus for controlling the electric machine of the type described above.
In the field of motor vehicle drive technology it is generally known to use an electric machine as the single driver or together with the drive motor of a different type (hybrid drive). Hybrid drives are typically comprised of a combination of an internal combustion engine and at least one electric machine as well as the associated energy accumulators in the form of fuel tanks and a battery. There are different types of hybrid drives, wherein two basic structures are differentiated, specifically serial and parallel hybrid drives. A combination of the two structures is also referred to as a power-split hybrid drive.
The serial hybrid drive is distinguished by a series connection of energy converters. This requires, for example, two electric machines and an internal combustion engine. One of the electric machines is operated as a generator and the other electric machine is operated as a motor. The internal combustion engine itself is not connected to a drive train of the motor vehicle. It charges the battery via the electric machine which is operated as a generator and/or directly makes available the required electrical energy to the electric machine which is operated as a motor. The power which is necessary to drive the motor vehicle is therefore transmitted exclusively from the electric machine which is operated as a motor to the drive train.
The parallel hybrid drive is distinguished by the fact that both the electric machine and an electric machine can pass on their respective mechanical power to a drive train. By means of a mechanical coupling of the two machines to the drive train their respective power can be added. The possibility of adding power in this way permits the two machines to be given relatively small dimensions without disadvantages occurring in respect of driving performance for the motor vehicle.
There are different types of implementing parallel hybrid drives. One possibility is to connect the electric drive directly to a crankshaft of the internal combustion engine (crankshaft-starter generator) or to connect it to the internal combustion engine by means of a belt drive. Both drive machines can therefore be used jointly or individually for propelling the motor vehicle. The electric machine can additionally optionally be operated as a generator or as a motor. The power-split hybrid drive is distinguished by a combination of the principles which have already been presented. The use of a power-split transmission (planetary transmission) permits part of the power of the internal combustion engine to be passed on directly, i.e. mechanically, to the drive train, while the remaining part of the power is converted into electrical energy via a generator. This electrical energy can in turn optionally be stored in a battery or passed on directly to an electric machine arranged downstream of the transmission. In the case of a power-split hybrid drive, both the electric machine and the internal combustion engine are used for propelling the motor vehicle.
Power electronics are typically used to control electric machines in a motor vehicle and, in particular, in a hybrid drive. These power electronics have an inverter which converts a direct voltage/direct current of the (high-voltage) battery located on board the motor vehicle into an alternating current. The power electronics conduct a high voltage of typically 60 volts.
Inverters have a series of power switches with which the individual phases (U, V, W) of the electric machine are optionally connected to a high potential, referred to as the intermediate circuit voltage, or to a low reference potential, in particular ground. Alternating voltages are therefore made available at the three phases of the electric machine by using the power switches. The power switches are controlled by a control unit which calculates a setpoint operating point for the electric machine as a function of the driver's request (accelerating or braking).
In a fault state or standby state/state of rest, the inverter cannot output any energy of the rotating electric machine into a direct voltage part of the system. The inverter is therefore switched into a safe state in order to prevent damage to electrical components. Essentially three different switch-off methods or operating modes are know from the prior art. In a short-circuit mode, all the switches which are connected to the load potential are closed and all the switches which are connected to the high potential are opened. In the other operating mode, referred to as open-circuit mode, all the switches of the inverter are opened.
In the case of a rotating electric machine, an alternating voltage is induced in the three phase sections of the electric machine. In the short-circuit mode, an alternating current flows in the phase sections of the machine via the closed path, which is owing to the induced alternating voltage. This gives rise to a sudden change in the machine torque, which can have a disruptive effect on the behavior of the vehicle. In the open-circuit mode, an alternating voltage is also induced. The level of the alternating voltage is dependent on the machine characteristic and the rotational speed of the electric machine. If the alternating voltage is higher than the intermediate circuit voltage, a current flows into the direct voltage part of the circuit. Since energy is transferred into this part, energy accumulators and other electrical components can be overloaded or damaged.
Since neither a sudden change in the machine torque nor a flow of current into the direct voltage part of the circuit is desired, it is known to switch either into the short-circuit mode or into the open-circuit mode as a function of the rotational speed, the machine characteristic and the level of the intermediate circuit voltage. However, the calculations for switching over between these two operating modes, which are carried out on the basis of the machine characteristic and the information from a rotational speed sensor system, are complex. Furthermore, the installation of the rotational speed sensor system on an electric machine entails additional costs.